Core A, the Human Studies-Biospecimen Collection Core, provides support for the operations of the Program Project through the collection offecal samples from twin pairs who are either concordant obese (BMI^35) or concordant lean (BMI 18.5-25). It takes advantage of an ongoing prospective study of a midwestern cohort of 1,800 female like-sex twin pairs, that has achieved high participation and retention rates: twins are being followed from mid-adolescence (median age 15)and will be of projected ages 25-35by the time of completion of our proposed five-year study. Specific Aims of the core are: Aims 1,2: Totrace and conduct telephone screening interviews with obese twins (BMI30) and their cotwins, aged 25-35, and then collect fecal samples from eligible female like-sex monozygotic pairs (year 1:N=20 pairs) and female like-sex dizygotic pairs (year 2:N=20 pairs) - 10concordant obese, 10concordant lean for eachzygosity group ,with 50%of each cell African-American, as well as from the biological mothers of these twins. Exclusionary criteria will include non-vaginal delivery, recent antibiotic use, premature delivery of the pair or birth complications resulting in perinatal care in a hospital's intensive care environment, and certain medical conditions. Aims 2-4: To obtain repeat fecal specimens from the MZpairs and mothers at one- month follow-up (year 1), 12month follow-up (year 2), 24month follow-up (year 3),36 month follow-up (year 4), and 48 month follow-up (year 5), and as well as an additional fresh fecal sample for isolation of strains of Bacteroides thetaiotamicron and Methanobrevibacter smithii from 8 MZpairs and their mothers (years 1 and 3).The sample collection protocol will be based on the core director's experience in remote collectionof samples from over 20,000 twins and relatives in ongoing research studies. Obesity represents a major and growing public health problem. The availability of a large cohort of twin pairs with repeated BMI assessments (byboth self-report and informant - corwin or parent report)from baseline assessment onwards, from whom over 500obese twins have already been identified, offers a unique opportunity to investigate host genotype, microbial, and host x microbial effects on obesity risk.